There was another post regarding this issue. I have a 69 Charger 440 auto. It has a rumbling at the tranny hump. I do know for certain that one of the flex plate screws is stripped (heli coil). Could that be it?

trans mount, bad u-joint, hitting exhaust pipe. Loose torque convert bolys will sound like a bad rod,,,bang bang bang as the things turns, once you are up to speed it might even stop. I'd jack the car up and takke a look. Jack the trans up a little and see if the mount is broken...grab the drive shaft and push it up down back and forth, look and see if anything is hitting the exhaust.

Fix your converter and make sure that the transmission is not rubbing on the hump. Take the driveshaft out of the transmission or while you do the flexplate check the u joints. Just because they feel tight doesn't mean that they are good.